Prior to the present invention, positive photoresists were generally based on moisture dependent chemistry utilizing a base polymer such as a novolak resin and a substituted 1,2-diazonapthoquinone as a dissolution inhibitor to render the novolak resin insoluble to development by dilute aqueous base, as shown by L. F. Thompson et al., Introduction to Microlithography, A.C.S. Symposium Series 219, American Chemical Society (1983) page 111. Upon irradiation, under moisture containing ambient conditions, the diazonaphthoquinone undergoes rearrangement and those portions of the resist which have received irradiation by image-wise exposure are rendered soluble to dilute aqueous base. While these photoresists are widely used, they are generally limited to the near UV above 366 nm based on the absorption characteristics of the diazonaphthoquinone. Although the quantum yield of the diazonaphthoquinone system is limited to a maximum value of 1.0, that is no more than one molecule of photosensitive composition can react per quanta of actinic light absorbed, its actual effective quantum yield has a value of only about 0.3.
Another moisture-dependent system utilizing ketals or acetals as the dissolution inhibitor in combination with a novolak resin is shown by Smith et al., U.S. Pat. No. 3,779,778. Smith et al., employs a photolyzable acid progenitor such as a chlorine substituted triazene, which, upon exposure to actinic irradiation, generates an acidic material which can catalyze the hydrolysis of the dissolution inhibitor.
Even though moisture-dependent positive photoresists systems are used extensively in the manufacture of high density electronic circuits, the successful implementation of such microlithographic processing requires a constant monitoring of the ambient atmospheric moisture to maintain a sufficient concentration of the water vapor needed to allow chemistry inherent in the reactions to proceed satisfactorily. For example, as shown by J. Pacansky et al., Photochemical Decomposition Mechanisms for AZ-type Photoresists, I.B.M., J. Res. Develop. 23, No. 1 (January 1979) pages 42-54, and particularly page 50, the importance of water is illustrated in the proper functioning of the orthonaphthoquinone diazide. In instances where the naphthoquinonediazide is photolyzed in the absence of water, the intermediate ketene reacts rapidly with the novolak resin to form a carboxylic acid ester which can lead to negative tone images. However, irradiation in the presence of ambient water results in the production of a 3-indene carboxylic acid which can be solubilized as the salt in the basic developer. As discussed by J. March in Advanced Organic Chemistry, Reaction Mechanisms and Structures, McGraw Hill Book Co., New York, Second Edition (1977) pages 345, acetals, ketals and orthoesters are readily cleaved by dilute aqueous acids.
As a result of moisture dependency, severe photolithographic problems can arise in particular situations, if moisture dependent positive photoresists are used under moisture free conditions. It has been found, for example, that if a moisture dependent positive photoresist is used with a Model 1100 wafer-stepper of the Ultratek Company of Santa Clara, Calif., or the Model 946SOR wafer-stepper of the Aste Company of Woodlawn Hills, Calif., a serious problems can arise in the fabrication of high density electronic circuits. The afore-mentioned instruments require a constant stream of a dry inert gas, between the lens and the surface of the photoresist in order to properly gauge the distance between the lens and the photoresist surface. As a result, it has been found that the speed of a moisture dependent photoresist can be impaired or rendered inoperable depending upon the period of time the wafer-stepper is used.
Recently, as shown by Ito et al., U.S. Pat. No. 4,491,628, Crivello, U.S. Pat. 4,603,101, and Narana et al., U.S. Pat. No. 4,663,269, incorporated herein by reference, chemically amplified positive photoresists systems utilizing acid generating onium salts have been developed, using t-butylester or t-butylether substituted materials which do not require the presence of moisture to function. As is well-known to those skilled in the art, such acid catalyzed cleavage of t-butylesters or t-butylethers result in an elimination reaction and the production of isobutylene and the corresponding acid or phenol. Accordingly, positive photoresists, such as Crivello, Ito et al., or Narano et al., can be readily employed either under substantially anhydrous conditions, if desired, or, optionally, under atmospheric or ambient conditions in a conventional manner. These t-butylester or t-butylether substituted polymers can be used in combination with an arylonium salt which generate a strong acid upon irradiation. These positive photoresists, having acid labile t-butylester or t-butylether groups, must be heated to deblock the acid labile groups. The deblocked portions of the polymer are rendered soluble in aqueous base, or insoluble in an organic solvent.
Even though the positive photoresists of Crivello, Ito et al., or Narana et al. can be utilized in the substantial absence, or optionally in the presence of moisture, these thermally deblocked photoresist systems require the use of special polymers. These special polymers having pendant t-butylester or t-butylether groups, or such groups along the polymer backbone, are at present not commercially available and therefore economically unattractive.
An object of the present invention, therefore, is to provide a positive photoresist system which can be used either in the substantial absence, or in the presence of moisture.
A further object of the present invention is to provide a positive photoresist system which incorporates at least a major amount by weight of a commercially available organic resin.
An additional object of the present invention is to provide a positive photoresist system which uses an effective amount of an arylonium salt which photo decomposes to a Bronsted acid.
The present invention is based on the discovery that commercially available novolak resins can be heated under particular conditions in the presence of a strong acid, such as an acid having a pKa of less than 0, without a significant degree of cross-linking occurring in the novolak. This result is quite surprising since, as taught by David Gould, Phenolic Resins, Reinhold Publishing Company, New York (1959) page 32, novolaks are unstable at elevated temperatures in the presence of a strong acid. This is confirmed by Newman, U.S. Pat. No. 4,708,925, which shows that crosslinking of a novolak can occur if heated in the presence of a Bronsted acid generated by an irradiated arylonium salt. A negative, instead of positive, image can be formed. Negative-tone images in novolak resin systems resulting from acid-catalyzed thermally activated reaction are also discussed in SPIE, Vol. 771, "Advances in Resist Technology and Processing", IV (1987) p.321.
Unexpectedly, we have discovered that a novolak resin can be heated in the presence of a strong acid as previously defined without a significant degree of cross-linking occurring in the novolak, if it is heated in the presence of a simple compound, as defined hereinafter, having a molecular weight of less than 1000 and thermally labile t-butylester or t-butylether groups. Simple compound deblocking occurs in the presence or absence of moisture. Upon deblocking, the simple compound and the novolak resin become soluble in aqueous base. The novolak resin does not cross-link to a significant degree, even though it is heated in the presence of a strong acid during deblocking. Accordingly, there is provided by the present invention, a positive photoresist which can be photopatterned in the presence or absence of water. The positive photoresist comprises a combination of a water or aqueous base insoluble simple compound having acid labile groups, such as t-butylester or t-butylether groups, a major amount by weight of an aqueous base soluble organic polymer, such as a commercially available novolak resin, and an arylonium salt.